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72 Decision Model and Notation, v1.5 
 
 
The character is the initial letter of the defined hit policy (Unique, Any, Priority, First, Collect, Output order or 
Rule order). Crosstab tables are always Unique and need no indicator.  
The hit policy SHALL default to Unique, in which case the hit indicator is optional. Decision tables with the Unique 
hit policy SHALL NOT contain overlapping rules.  
Tools may support only a nonempty subset of hit policies, but the table type SHALL be clear and therefore the hit 
policy indication is mandatory, except for the default unique tables. Unique tables SHALL always be supported.  
  
8.2.11.1 Single and multiple hit tables  
A single hit table shall return the output of one rule only; a multiple hit table may return the output of multiple 
rules (or a function of the outputs, e.g., sum of values). If rules are allowed to overlap, the hit policy indicates 
how overlapping rules have to be interpreted.  
The initial letter for hit policy also identifies if a table is single hit or multiple hits.  
A single hit table may or may not contain overlapping rules but returns the output of one rule only. In case of 
overlapping rules, the hit policy indicates which of the matching rules to select. Some restrictions apply to tables 
with compound outputs.  
Regardless of whether a single or multiple hit policy is used, some columns in a decision table may be designated 
as rule annotations. Rule Annotations contain text that is not returned as part of the expression results, and they 
are ignored for purposes of the hit policy validations described below. Although there is no standard mechanism 
to access the annotations of the matched rules in a decision table at execution time, implementations may use the 
annotations for auditing, debugging, logging, documentation, analytics, consumption by down-stream systems, or 
for other purposes.  
Single hit policies for single output decision tables are:  
1. Unique: no overlap is possible, and all rules are disjoint. Only a single rule can be matched. This is the 
default.  
2. Any: there may be overlap, but all the matching rules show equal output entries for each output (ignoring 
rule annotations), so any match can be used. If the output entries are non-equal (ignoring rule 
annotations), the hit policy is incorrect, and the result is undefined.  
3. Priority: multiple rules can match, with different output entries. This policy returns the matching rule 
with the highest output priority. Output priorities are specified in the ordered list of output values, in 
decreasing order of priority. Note that priorities are independent from rule sequence.  
4. First: multiple (overlapping) rules can match, with different output entries. The first hit by rule order is 
returned (and evaluation can halt). This is still a common usage because it resolves inconsistencies by 
forcing the first hit. However, first hit tables are not considered good practice because they do not offer a 
clear overview of the decision logic. It is important to distinguish this type of table from others because 
the meaning depends on the order of the rules. The last rule is often the catch-remainder. Because of this 
order, the table is hard to validate manually and therefore has to be used with care.  
  
A multiple hit table may return output entries from multiple rules. The result will be a list of rule outputs or a simple 
function of the outputs.  
Multiple hit policies for single output decision tables can be:  
5. Output order: returns all hits in decreasing output priority order. Output priorities are specified in the 
ordered list of output values in decreasing order of priority.  
6. Rule order: returns all hits in rule order. Note: the meaning may depend on the sequence of the rules.  
7. Collect: returns all hits in arbitrary order. An operator (‘+’, ‘<’, ‘>’, ‘#’) can be added to apply a simple 
function to the outputs. If no operator is present, the result is the list of all the output entries. Collect 
operators are:  